1. Field of the Invention
The present invention relates a method of manufacturing photoelectric conversion device, and more particularly to a technique of forming a contact hole in a process of manufacturing a photoelectric conversion device.
2. Description of the Related Art
In recent years, lower-cost digital cameras and digital video cameras with higher image qualities have prevailed thanks to advanced photoelectric conversion devices. Photoelectric conversion devices are classified into the CCD type, MOS type, and the like. A MOS photoelectric conversion device includes a pixel region and a peripheral circuit region. The pixel region includes a photoelectric conversion unit, a transfer transistor and an amplifying transistor. The peripheral circuit region includes a MOS transistor and processes a signal output from the pixel region and drives the photoelectric conversion device. The pixel region and peripheral circuit region can be manufactured in a common step by the CMOS manufacturing process.
The MOS transistor can operate quickly by adopting a salicide (to be referred to as a “silicide layer”) structure in which a semiconductor compound of a high melting point metal is selectively formed on the source, drain, and gate electrode of the MOS transistor in the peripheral circuit region. However, the use of the silicide layer for the photoelectric conversion unit in the pixel region increases the leakage current of the photoelectric conversion unit, degrading the photoelectric conversion characteristic.
Japanese Patent Laid-Open No. 2001-111022 discloses an arrangement in which the salicide structure is used only in the peripheral circuit region to suppress the leakage current of the photoelectric conversion unit.
In some conventional active pixel solid-state image sensing devices typified by a CMOS image sensor, pixels have a global electronic shutter function to make the start times and end times of photo charge accumulation coincide with each other in a plurality of pixels arrayed in matrix.
To implement a solid-state image sensing device having the global electronic shutter function, the pixel region requires a photoelectric conversion unit which performs photoelectric conversion, and a charge holding unit which holds charges generated in the photoelectric conversion unit for a certain period. The charge holding unit holds charges till reading after the end of accumulation in a given pixel. If light enters the charge holding unit and charges are generated by photoelectric conversion at the P-N junction adjacent to the charge holding unit, the charges act as light leakage noise, degrading the image quality.
To solve this problem, Japanese Patent Laid-Open No. 2007-294531 discloses an arrangement in which a light-shielding film is formed to shield the charge holding unit from light without shielding the photoelectric conversion unit from light. The light-shielding film prevents light from entering the charge holding unit, thereby suppressing degradation of the image quality.